a. Field of the Invention
The invention relates to acoustic distance measuring gauges, and more particularly to an acoustic method and apparatus for measuring extremely small distances between a gauge head and a surface.
b. Prior Art
Acoustic distance gauges are known. Some rely on pulse-echo measurement, others rely on acoustic pressure. An example of a pulse-echo gauge relying on the magnitude of reflected signals for use in thickness measurements, is in U.S. Pat No. 3,918,296, while examples of pulse-echo gauges relying on phase shift may be found in U.S. Pat. No. 2,268,643 to Crosby and U.S. Pat. No. 3,500,301 to Meier. An example of a distance gauge relying on acoustic pressure may be found in U.S. Pat. No. 4,175,441 Urbanek, Kren and Wheeler.
With regard to the aforementioned Meier patent, ultrasonic waves are transmitted from a source to a target web where the waves are reflected back to a receiver. Variations in phase of the returned waves indicate distance to an object. Meier teaches that the minimum sonic path length between transducers and the target web should be at least four wavelengths, lest non-linearities due to sonic near-field effects become too severe. Using an operating frequency for ultrasonic waves of 40 kHz, Meier is able to detect unique path length changes within one-half of a wavelength, or 0.165 inches in air at 80.degree. F.
Crosby teaches use of a distance finder wherein phase changes for waves reflected from a target of different frequencies, primarily radio frequencies, are eombined. The amplitude off the resultant is indicative of distance to a target. While Crosby deals with less than one wavelength in an example relating to radio frequencies, he suggests path lengths of plural wavelengths for sound waves.
While these prior art distance gauges are useful, there is a need for a non-contacting distance gauge which can measure variations in distance of a micron and less. Such measurements are needed for checking flatness of optical components, semiconductor wafers, and the like.